The present invention relates to a sampling pump for a chemical sensing system. More particularly the invention relates to a sensing system using a sniffing mode, alternately inhaling and exhaling in each pumping cycle to expose the sensor or other chemically responsive surface to doses of reference gas and analyte.
A large number of chemical and biological sensors are based on changes in the properties of a chemically sensitive material, such as changes in conductivity, surface charge or luminescence, that occur upon adsorption of analyte molecules. Analytes are, of course, the gas to be chemically analyzed. These changes can be monitored through physical methods and are related to the concentration of the analyte in the environment. In order to reduce the time associated with the diffusion of the analyte to the sensor site and to increase the amount of analyte seen by the sensor, sampling methods are used which force the air from the environment into direct contact with the sensitive polymer or other sensor material.
A typical present day configuration includes a material having physical properties that change when its surface is exposed to a gas containing certain chemical or biological species. These properties may be optical, electrical or mechanical, for example. A gas sampling system is used to bring fresh gas samples into contact with the surface of the material. Then, a read-out and signal processing system of electronics is used to convert the physical change to a useful output.
While these sensors have been demonstrated to work over a short time basis, they have been found to be adversely affected by long term drift that limits their practical use. The baseline drift frequently exceeds the minimum detectable signal by orders of magnitude, so that a sensitive technique is rendered essentially useless. It would be of great advantage to the art to greatly reduce the effects of baseline drift in gas sensing systems.
It would be another great advance in the art if a system would be developed to enable or significantly enhance the use of a wide variety of chemical and biological sensing techniques now not useful due to an inability to distinguish between the response to an analyte and the effect of baseline drift.
Other advantages will appear hereinafter.
It has now been discovered that the above and other objects of the present invention may be accomplished in the following manner. Specifically, the present invention provides a sampling system for a chemical sensor that works in a sniffing mode, that is, it produces an alternating flow pattern that alternately exposes the sensor head to a dose of reference gas and then to a dose of analyte gas during each sampling cycle.
The flow pattern during the exhaling phase is sufficiently powerful to insure a fresh sample at each intake phase of the sniffing cycle. The principle, used by all breathing animals, has not been previously suggested or used for active gas sampling.
The sampling system also functions in a purge mode to restore the baseline output of the sensor. The present invention permits the use of signal processing techniques that suppress background and sensor baseline drift, and thus significantly improve the sensitivity and usefulness of chemical sensors.
The diaphragm pump used in the present invention operates in two different modes to accomplish the goals of the invention. In a DC mode, the pump produces a gas flow in one direction through a filter or other cleaning device. This cleaned air is further used as a reference gas for the second mode, known as the AC operation regime.
The second or AC operation mode performs the sniffing function by causing the direction of flow to alternate during each cycle. Gas flow in this regime is analogous to the electric current in an AC electrical circuit.
In the present invention, operation sequence first includes filling the whole sampling system with cleaned reference gas using the above referenced pumping cycle.
An intake phase of the sniffing mode follows, where a fresh sample of analyte gas from the environment of interest is brought into the sensing head. Outside air is kept from getting into the pumping chamber, avoiding contamination of the pump.
An exhaling cycle follows. The diaphragm action pushes the sample out of the sensing chamber and fills the sensing chamber again with the reference air from the buffer and the pump chambers.
The cycle starts over again by inhaling another sample.
In its simplest embodiment, the mesosniffer system consists only of a diaphragm pump working in the AC mode and a chemical sensor. The AC mode pump moves air back and forth across a sensor surface which is responsive to the desired analyte. The interaction between the analyte and the sensor is assumed to have an irreversible component, so that all the absorbed analyte is not desorbed. This causes a slowly increasing output which cannot be distinguished from baseline drift. AC signal processing eliminates the effect of the slowly changing baseline and provides an output proportional to the concentration of analyte.